If I open a map, and click along the tree. I press enter and I get a list of elements that I can insert. What defines the list, is it the techinfo.xsd file? How do I add or hide elements? I am looking specifically at adding the appendix element. I have looked in the default stylesheet and it is an element that is listed in there.
Yes, the "quick tags" menu (showing what can be inserted at the current caret location by pressing Enter) is determined by the DTD/XSD. It won't let you insert anything that isn't valid according to the doctype.
If you want to add an element, you would do that via a DITA specialization. However, there's a bit of overhead involved in that, it's not as simple as just popping a new tag name into an element content model. The Help Center has information about DITA specialization if you search for that term.
An easier approach would be to use one of the already defined topics, with a specific label in the "outputclass" attribute. For example, you might use something like this:
Thanks Clay. That sounds like something to think about. I am disappointed that it is available in SMA but not in TIA. I thought if the element was already in the style sheet that I could see if it is available to be inserted into a ditamap.